Attributing extreme forest drought stress to climate change and effects of converting forests in Germany

The extreme drought of 2018 has triggered widespread decline of coniferous trees in Germany. It is unclear how much climate change contributes to these observed impacts and if the currently ongoing forest conversion from coniferous to broad-leaved trees is sufficient to alleviate future forest drought stress. We use a novel attribution framework and model simulations to attribute drought stress to climate change and assess the effectiveness of forest conversion as an adaptation measure. Climate change adds 18 (+/- 2, 13 %) days of forest drought stress in 2018 and more than halves the return period of a 2018 event (408 to 168 years). The return period increases by 51 years and decreases by 69 years for a 2018 event under moderate (SSP126) and severe (SSP585) climate change respectively. We show that converting vulnerable conifer forests reduces drought stress of a 2018 event by 16 days (12%). However, increased forest drought stress under high emission scenarios requires further adaptation.